Measurement of Computer-Generated Plates.

This research study builds on GATF's 1995 study of computer-to-plate imaging systems. The 1996 research focused on the measurement of computer-generated plates for the purpose of process control. Two different approaches to measuring printing plates were tested: A digital file containing a test element was imaged with the press form file. The finished printing plates were measured with a variety of densitometers. Several manufacturers of platesetters, printing plates, and measuring instruments participated in the study. This research found that: 1. The Digital Plate Control Target was an effective exposure control device for the printing plates used in this study. It was also useful for measuring plate resolution, directional effects, and dot area range. 2. Reflection readings from the printing plates did not, in many cases, provide realistic dot areas for the highlights and quartertone values. Their readings for the midtones and three-quartertone values were better correlated with the printed results. 3. Overall, the five reflection instructions agreed with each other. However, at certain tone values some instruments were significantly better correlated with press results than others. 4. The inaccuracy of dot area calculations from reflection readings was of a magnitude that would be visually apparent. This condition was much worse for underexposed (vs. normal or overexposed) plates, and slightly worse for polyester (vs. aluminum) plates. Only slight differences were found between 85- and 200-line screens. Consistent with last year's study, several concerns were found that should be weighed by commercial printers who are considering buying computer-to-plate (CTP) systems. In some instances, a raster image processor (RIP) overrode PostScript headers. Some platesetters exhibited directional bias, as shown by oval (rather than round) star target centers. Others had difficulty imaging vertical and horizontal microlines at the same thickness. Some systems had problems imaging checkerboard patterns. The photomacrographs showed that some plates could hold more detail than others. Certain plates produced a higher degree of banding in vignette patterns.